It is common practice to inspect work pieces subsequent to production on a coordinate positioning apparatus, such as a coordinate measuring machine (CMM), in order to check for correctness of predefined object parameters, like dimensions and shape of the object.
In a conventional 3-D coordinate measurement machine, a probe head is supported for movement along three mutually perpendicular axes (in directions X, Y and Z). Thereby, the probe head can be guided to any arbitrary point in space of a measuring volume of the coordinate measuring machine and the object is measurable with a measurement sensor (probe) carried by the probe head.
In a simple form of the machine a suitable transducer mounted parallel to each axis is able to determine the position of the probe head relative to a base of the machine and, therefore, to determine the coordinates of a measurement point on the object being approached by the sensor. For providing movability of the probe head a typical coordinate measuring machine may comprise a frame structure on which the probe head is arranged and driving means for moving frame components of the frame structure relative to each other.
For measuring surface variations, both measurement principles based on use of tactile sensors and of optical sensors are known.
In general, to provide a coordinate measuring machine with high measurement precision, its frame structure is therefore usually designed to have a high static stiffness. In order to achieve a stiff and rigid machine design, the frame structure or at least parts of it, is often made of stone, such as granite. Besides all the positive effects like thermal stability and good damping properties, the granite or other stiff materials also makes the machine and the movable frame elements quite heavy. The high weight on the other side also requires high forces for a decent acceleration.
However, weight reduction is a main topic relating to the designs of coordinate measuring machines, as if the machine components are built comprising less weight (and less stiffness) faster positioning of respective components can be achieved by causing fewer force affecting the coordinate measuring machine. On the other hand the influence of machine vibrations and torsions caused by reduced stiffness and (faster) movement of the machine components increase with weight reduction of these parts. Thus, uncertainties of derived measurement values and errors occurring from such deformations and vibrations increase accordingly. Therefore, especially with view to weight reduction but also for conventional machines, an accurate error handling is an important aspect.
For both approaches (heavy and light weight) an initial calibration procedure of the respective CMM is necessary particular for determining static and repeatable errors of the respective system. For maintaining stable and accurate measuring requirements, such a calibration preferably is to be executed in defined intervals due to taking account of external influences affecting the measuring system over time, e.g. changes of environmental parameters (temperature, humidity etc.) or mechanical impacts.
The calibration of a CMM may provide an improvement of a model which describes the static and/or dynamic behaviour of the CMM under certain conditions. Thereby, current calibration parameters may be used for actualising the defined model in order to more precisely—and adapted to current conditions—describe the behaviour of the CMM.
Typically, a so called compensation map is derived by the calibration procedure, wherein the map provides a compensation of each measuring value, which is acquired by measuring a measuring point of an object. Such a map may be designed as a kind of look-up table, i.e. for every coordinate or for defined coordinate steps of each axis of the CMM a corresponding compensated value is provided and an originally measured value is replaced by the compensated one. Alternatively, specified equations are determined and the equations are applied to measured position values for calculation of corresponding corrected values, thus providing a kind of compensation map.
There are several techniques and methods known for respective calibrations of a coordinate measuring machine. According to one known procedure, a distance measuring device—particularly a device providing distance measurement by a laser beam—is located on the base of a CMM and distances are measured to a target, the target being attached to the probe head of the CMM and being moved along a defined path, whereby respective distances and machine coordinates are derived at designated positions along the path. Geometric errors of the measuring machine are determined on basis of the measured distances and machine coordinates are acquired by the machine. Such a method for instance is disclosed in the European patent application EP 1 990 605.
Disadvantageously, for determination of precise calibration parameters for the whole measuring volume of the CMM, the laser device has to be set successively to different positions within the measuring volume and a number of measurements for several directions of the laser beam have to be performed. Thus, a highly educated operator, which is specifically trained on the calibration procedure, has to relocate the laser device several times accordingly and has to control and/or monitor the procedure as a whole. Therefore, as the proposed procedure needs to be monitored and manually controlled, that procedure is very time consuming and quite expensive.
A similar approach, however located in the different technical field of processing machines and thus relating to quite different requirements concerning machine design and measuring precision, is known from the EP 2 390 737, wherein an emitter is located at the tool head and reflectors are positioned at the processing machine.
Another calibration method for a CMM is known from German patent DE 199 47 374 (so called “Etalon Laser Tracer”). According to that method again a target (i.e. a reflecting member) is arranged at the probe head of the CMM. Moreover, at least one laser tracker is provided for determining the position of the target and to continuously track the target. The target is moved according to a predetermined path and measurements are performed at designated positions, wherein a position of the target is determined by the laser tracker and respective coordinates are derived by the CMM. That procedure is repeated several times, wherein each time the laser tracker is located at a different position and measurements are performed at identical designated positions.
Geometric deviations and corresponding correction values are derived on basis of the CMM coordinates and corresponding positions determined by the laser tracker. A hardware setup proposed to be used for that calibration method is described in more detail in EP 0 919 830.
Here, too, as the calibration as a whole is a quite complex procedure (setting up the laser tracker at different position and performing respective measurements at identical CMM-positions), one big disadvantage of that calibration method is the facts that the calibration is to be executed by a specifically educated person. Moreover, providing that method is comparatively very expensive and time consuming, as at least one precise laser tracking device is needed and that device is to be placed at different precisely-known positions. Alternatively, more than one laser tracker is provided, which makes the method even more expensive.